KANDAHAR, Afghanistan (Reuters) - Growing violence in the southern province of Kandahar ahead of Afghanistan's presidential election next week highlights a rift between Pashtun tribes that could tip the country back into civil war.

The fate of Kandahar, birthplace of the Taliban insurgency, is crucial. Some Afghans fear a loss in the vote for Zalmai Rassoul, a candidate close to President Hamid Karzai, could push Kandahar's powerful Pashtun tribes into rejecting rule from Kabul, some 500 km (300 miles) away. Others fear the same outcome if he wins.

Such a revolt would have the potential to split the rugged nation of 30 million people, already divided by fierce loyalties beyond the Pashtuns among such tribes as the Tajiks, Uzbeks and others.

The two presidential frontrunners from Kandahar are Rassoul, backed by Karzai's brothers, and Ashraf Ghani, a former World Bank official. Both are Pashtuns.

"If Kandahar is peaceful, Afghanistan is peaceful," provincial governor Tooryalai Wesa told Reuters. "The key of Afghanistan is here. The history and politics of Afghanistan have always belonged to Kandahar."

The violence in Kandahar comes despite the governor doubling the size of the provincial army, driving the Taliban to the edge of the region.

But now the militant group, which has vowed to disrupt the April 5 vote and return to power, is once again gaining momentum. Just last Friday it killed the Kandahar governor's chief of staff and wounded the deputy governor in a bombing.

That blast followed an earlier attack by suicide bombers targeting an intelligence agency building.

Tribal leaders in Kandahar say the Taliban insurgency has been galvanised by Karzai's 12 years in office because of his family's growing wealth and power, which have fuelled rivalry between regional elites. Karzai is from the powerful Pashtun Popalzai tribe, Ghani from the Ahmedzai tribe.

"People in Kandahar are tired of exclusive rule by the president's brothers, while other tribes have been suppressed by the Popalzai tribe," said Haji Ehsaan, an elder of another Pashtun tribe, the Noorzai.

With Karzai legally barred from a third term, many Afghans see his family's public backing of Rassoul, a former foreign minister, as a move to tighten its grip on Kandahar.

All nine presidential candidates are Pashtun except Abdullah Abdullah, a frontrunner along with Rassoul and Ghani, who has a Pashtun father and a Tajik mother and is famous for his closeness to assassinated Tajik militia leader Ahmad Shah Massoud.

A successful vote in April would mark Afghanistan's first democratic transfer of power and signal prospects for stability after most foreign troops withdraw at the end of the year.

But only a win for Rassoul would ensure this stability, his supporters said.

"He is a Pashtun from Kandahar and has the support of the Karzais," said the chief of the Kandahar provincial council, Haji Sayed January "If someone other than Rassoul wins, the people of Kandahar will not accept that and tribal tensions will grow."

FAMILY ISSUES

Karzai's half-brother, Shah Wali, is the leader of the Popalzai tribe, and the businesses of the president's brothers, Mehmood and Qayum, have flourished during his tenure.

The two men said they planned to fight the Taliban's growing influence in Kandahar by fighting economic deprivation, stiffening the justice system and rooting out corruption.

"I am passionate about what we believe to be the problems of the Afghan people," said Mehmood, whose brother, Qayum, withdrew from the presidential election to support Rassoul.

But even in Karzai's family there are cracks.

One fierce critic is his cousin, Hashmat, who said corruption and injustice among the current leaders were driving disenchanted citizens into the arms of the Taliban.

The Taliban, in power from 1996 to 2001, are seeking to oust foreign forces and set up an Islamic state.

Hashmat said he was gathering tribal leaders from three southern provinces to back Ghani, the former World Bank official, who is popular with young people and women.

"Once the tribes or the elders decide (to support Ghani), it would make a big impact on the election," said Hashmat, speaking to Reuters at a villa where tribal leaders met to weigh election strategies, while a pet lion, a symbol of power and status in Afghanistan, paced the garden outside.

If Rassoul won, more people would join the Taliban, Hashmat said, strengthening the insurgency as foreign troops pull out.

But it will be hard to convince the people of Kandahar that the elections will be fair.

Most of them expect a repeat of the corruption that marred the presidential vote in 2009, when about 40 percent of votes cast in Kandahar were suspected to be fraudulent.

"What was happening before will happen on the same scale," said Abdul Hadi, chief of Kandahar's Independent Electoral Commission, speaking in his office beside a graveyard of Soviet tanks left behind from the Russian invasion of 1979.